[ruby/mutex_m] Bump up 0.2.0

https://github.com/ruby/mutex_m/commit/635da25a09

lib/mutex_m.rb

@@ -40,7 +40,7 @@
 #
 module Mutex_m
 
-  VERSION = "0.1.2"
+  VERSION = "0.2.0"
   Ractor.make_shareable(VERSION) if defined?(Ractor)
 
   def Mutex_m.define_aliases(cl) # :nodoc: